This invention generally relates to balun transformers that may be implemented on a printed circuit board or in surface mount packages, and more particularly to a transformer layout suitably adaptable for use in microwave modules where the transformer is integrated in the printed circuit board.
A balun is a device designed to convert between balanced and unbalanced electrical signals, such as between, for example, a coaxial cable and a ladder line. Baluns may be considered as simple forms of transmission line transformers. The seminal work on transmission line transformers was done by Gustav Guanella in 1944. Guanella's transformer is described in U.S. Pat. No. 2,470,307. Guanella proposed coiling transmission lines to form a choke that would suppress the undesired mode in balanced-to-unbalanced matching applications. Guanella's transformer comprised a basic building block or a 1:1 balun. As is generally known, the choking reactance of Guanella's transformer isolates the input from the output. That reactance is usually obtained by coiling the transmission line around a ferrite core or by threading the line through ferrite beads. In general, the objectives are to have the characteristic impedance, Z0, of the transmission line equal to the value of the load RL, which is generally referred to as the optimum characteristic impedance, and to have the choking reactance much greater than RL (and hence Z0). By combining coil transmission lines in parallel-series arrangements, Guanella was able to demonstrate very broad baluns, with ratios of 1:n2 where n is the number of transmission lines. FIG. 1A shows a schematic for Guanella's 1:4 balun.
Baluns may be implemented in a variety of ways using different types of transmission line elements. For example, coaxial transmission line segments can be used to form baluns at relatively low frequencies. These types of baluns are, however, difficult to integrate with other elements of microwave circuits where surface mount design techniques are used. Where such techniques are employed, baluns may be formed on printed circuit boards. Such baluns may be fabricated from stripline or other conductor materials that is formed on a substrate of a printed circuit board in a known manner. In addition to stripline, microstripline or other technologies have been used to form the conductive traces. In general, surface mount designs require that components or devices are mounted on a printed circuit board having printed circuit traces on at least one surface of the board. The surface mount components connect to the terminals of the traces using known soldering techniques.
Turning now to FIG. 1B, there is shown a schematic circuit diagram of a prior art transformer. Due to the asymmetry cased by grounding at port 5, it is typically necessary for this balun transformer to function at the junction point where L1 is tapped. This point does not always have a zero potential for the differential signal. This usually leads to degradation in performance. The transformer of FIG. 1B is commonly used in diode double-balanced mixers normally followed by a Guanella balun transformer. U.S. Pat. No. 6,294,965 to Merrill et al. modifies the transformer of FIG. 1B but the summing function needed for mixers and to establish a DC supply point is not provided.
FIG. 1C shows a equivalent schematic circuit diagram of the transformer described in the '965 patent.
In surface mount applications it is desirable to have the various circuit elements and conductive traces desirably shaped to meet design demands. The present invention provides a balun transformer that may be implemented on a printed circuit board so as to advantageously allow more flexibility in the layout of the circuit elements.